prounciation of desu

Sometimes in the speaking desu is pronounced with a u and sometimes not when it is a the end of a sentence. How do I tell the difference?

You can pronounce it either way. The Japanese sometimes tend to skip a vocal to make the speech more fluent. But usually, desu is pronounced with the u, as this is the correct way to do it. However, in some situations, you may drop the u (not completely though, only pronounce it a little faded). For example: "Nan desu to" is fluently pronounced as "nan des to". U is usually dropped only in situations where desu is followed by something else. Hope this helps

Thanks, that helps a lot.

Japanese will never pronounce the u on the end. I work with them every day in tourism.